Recently attention has been directed towards providing a number of electrical power generators associated with rotary components of a prime mover such as a gas turbine engine. These generators are typically attached to rotating shafts on the high pressure and low pressure shafts of the gas turbine engine, rotating at different speeds and may be of different construction to satisfy the operation environment at respective locations. In such circumstances it will be understood that the generators produce electrical power at different frequencies, differing number of phases and with potentially different values of current and voltage. These differences in the electrical power produced by the respective generators mean that it is not possible to directly parallel two or more of the electrical machines as generators on a common electrical power distribution bus.
The conventional approach to paralleling of electrical generators is to condition the electrical power generated from each generator through a power electrical converter to feed a direct current (DC) power bus. Unfortunately a large amount of capacitance in the order of several thousand micro-Farads is required for such a power bus to maintain a steady value of bus voltage in the presence of distributing and switching loads, that is to say switching in and out of electrical generators as well as consumer device load requirements. It will also be understood that DC electrical power buses also provide a convenient distribution medium for exchanging energy between the respective rotating shafts of the prime mover machine, that is to say the gas turbine jet engine which may allow more efficient use of fuel within an operational episode with the prime mover such as a gas turbine engine in an aircraft during a flight cycle.
Traditionally land based electrical systems have utilised electrolytic capacitors which are particularly capacitance dense resulting in a size and weight benefit. However, this type of capacitor is not generally viable as an aerospace quality component because of limitations of operational temperature, capacitance stability, limited life and orientation limitations. In such circumstances, particularly in aerospace applications, less capacitive dense technologies such as film-foil are utilised such that a larger capacitor is required and therefore there is a significant weight and size penalty in order to achieve a practical arrangement. For example, if the required DC capacitor is in excess of 50,000 micro-Farad it will be understood with a typical practical film-foil capacitor having only 0.05 micro-Farads per square centimetre capacity then the total volume of capacitance required would be in excess of 1,000,000 cubic centimetres or 1,000 cubic metres and weight around 1000 kg. Clearly this is impractical and so limitations with respect to electrical power equalisation are a significant constraint upon appropriate application of multi power generator technologies. It will also be understood that large capacitors store very large amounts of energy which can be released in the event of a fault on a DC electrical network. This can give large fault currents of several thousands amps which are very difficult to control and result in significant transient forces being generated within the electrical distribution system. In such circumstances with large capacitors there is a significant risk of explosion or fire when the energy is rapidly released to a fault load.